I would still use illiterate in that case, and would argue that is the correct term at least according to Merriam Webster:
a : violating approved patterns of speaking or writing
b : showing or marked by a lack of familiarity with language and literature
Or even just at its most basic:
having little or no education
Thank you, I've only ever heard it used in the respectful sense among Black (and sometimes Native American) people. I get language evolves sometimes but it sucks they turn positive things into negative things, and I'd argue that's very intentional.